Phonon spectra of plutonium at high temperatures

Ab initio molecular dynamics calculations are used to investigate the vibrational properties of the high-temperature delta and epsilon phases of plutonium. We combine the local-density approximation (LDA)+U for strong electron correlations and the temperature-dependent effective potential method in order to calculate the phonon spectra of the two phases, as well as their dependence on temperature. Our results show that the epsilon phase can only be stabilized when temperature and correlations are simultaneously accounted for. We are also able to quantify the degree of anharmonicity of the two phases. While the d phase is fairly harmonic up to 1000 K, we find that the epsilon phase is strongly anharmonic, which explains why this structure dominates the phase diagram at high temperature.